The present invention relates to a method of, and a circuit for, estimating true data from distorted digital data signals which may have become distorted during transmission.
In systems involving the transmission of data, it is important to try to avoid the effects of data becoming sufficiently distorted so that falsing of the data occurs. For example, in a paging system if address data becomes distorted in transmission, then it is possible that the true addressee of the paging call is not alerted because the address information has become corrupted.
U.S. Pat. No. 3,479,457 discloses an apparatus for demodulating phase--or frequency--modulated signals by high speed coded signals. Faults, for example, degradations in the signal to be demodulated, are corrected by clocking the signal into a shift register at a higher frequency than the frequency of the signal to be demodulated. Then pairs of signals from the shift register are compared in an Exclusive-OR circuit and the result is fed to a second shift register having three stages. Different pairs of outputs of the second shift register are supplied to a majority logic circuit comprising three AND gates and three input OR gates connected to the output of each of the AND gates. If necessary this circuitry is duplicated to rectify further faults. A drawback to this known circuit is the amount of hardware involved in the majority logic stage if the number of samples is increased. For example, this cited specification states that if one stores five samples in the second shift register which now has five stages then to obtain a majority logic decision by considering different combinations of three samples, ten AND gates are required.
U.S. Pat No. 3,859,655 also discloses reforming of a signal by use a shift register and majority logic.
A drawback to these known systems is that an increasing number of circuit elements are required as the number of samples used in making a majority logic decision becomes greater. Accordingly, in situations where space and power consumption are of importance, such as in paging receivers, it is desirable to be able to make a reliable decision based on a smaller number of samples when estimating true data from distorted digital data signals.